Methods presently employed for obtaining and maintaining registration during the copying of film images from a moving continuous strip master to a moving continuous strip duplcate are many and varied. One method comprises exposure of the inner, emulsion surfaces of the master and duplicate to a vacuum as taught in U.S. Pat. No. 3,740,140 to Wolf, et al, as they are brought into registry and prior to irradiation by a light source. In such an apparatus, the enclosure in which the vacuum is created is partially formed by the moving master and duplicate film strips, thus requiring them to have a precise and known width with respect to solid side walls which form another portion of vacuum enclosure. As the duplicate and master are brought into registry, the vacuum created between the side walls and their respective confronting surfaces during registration results in atmospheric pressure on the opposite, non-confronting surfaces maintaining registration until the image on the master has been copied onto the duplicate during an exposure process.
In methods wherein vacuum is applied only to confronting surfaces of the master and duplicate, several problems have developed which limit the application of this potentially useful technique. One problem relates to tension variations between the master and duplicate film dispensing devices. In the technical arts, the term "shmoo" is used to define the ratio of tension on the film dispersed from a reel containing the strip master (T.sub.1) and a reel containing a strip duplicate (T.sub.2). As long as T.sub.1 equals T.sub.2 there is no problem. However, as the ratio T.sub.1 /T.sub.2 or shmoo varies from one, the amount of vacuum required to maintain registration during the exposure process varies. In systems wherein the sides of the master and duplicate partially create a vacuum-containing enclosure, the shmoo is limited to values close to 1 due to a necessity to maintain a vacuum enclosure of substantially known dimensions. It is also required that the vacuum be limited to a predetermined level so that atmospheric pressure on the non-confronting master and duplicate film sides will not prematurely force the confronting surfaces into registration or, in some configurations, expose an aperture through which the vacuum is applied to the external atmosphere, thereby destroying the integrity of the vacuum created within the enclosure. In addition, present vacuum heads for film positioning systems require that the width of the master and duplicate be substantially equal and known in order to maintain the vacuum integrity of the enclosure. These systems are therefore very sensitive to film edge imperfections and splices whenever there is a lateral offset at the splice junction because of the vacuum destroying gaps created between such edge imperfections and junctions and the side walls forming the vacuum enclosure. For the same reason present systems cannot accommodate film with sprocket holes. Another shortcoming of prior systems is that bending manner differences introduced by film of different thicknesses require elaborate and expensive tension control devices due to the limited shmoo which can be tolerated as explained above.
The vacuum head film positioning apparatus disclosed herein greatly reduces or eliminates all of the above problems by exposing all film surfaces to the same vacuum conditions prior to their registration and subsequent exposure of the non-confronting surfaces to atmospheric pressure. This is accomplished through use of a housing in which a vacuum is maintained. The master and duplicate are inserted into the housing so that both sides of each are exposed to the vacuum and they are removed with their confronting longitudinal surfaces in substantially continuous contact. The vacuum between the confronting surfaces in conjunction with atmospheric pressure on the non-confronting surfaces subsequent to removal from the housing is sufficient to maintain the film in registration until an exposure process has been completed.
In a particular embodiment, the housing is formed of two side walls and of continguous longitudinally extending rollers. Preferably, the rollers are formed with elastomeric surfaces. The master enters the housing through a first roller pair, the duplicate through a second roller pair and both are removed through a third roller pair which supplies the positioning necessary to bring the two film strips into contact with each other. Each roller is positioned so that film passing between any roller pair is in close contact therewith. In a particular embodiment, there are six rollers defining the circumference of the housing. The film removed through the third roller pair passes while still in registry, and is irradiated by, a light source in an exposure zone.
The apparatus provided herein overcomes the difficulties of the vacuum positioning heads used heretofore. Since both the master and duplicate film strips each pass through a roller pair, making sealing contact therewith, and have both sides thereof exposed to the same vacuum within the housing, a much larger shmoo can be accommodated because correct positioning of the film strip to the vacuum enclosure is no longer a requirement. In addition, problems related to splice mismatch, film edge imperfections and sprocketholed film become non-existant.